I recently got a set of Conquest/Starion wheels for my Z31.

The tires are mismatched and aren't in the best of shape. The first thing I noticed when putting the wheels on the car is that there is much more vibration in the steering wheel when going above 55 mph.

The stock wheels + new tires that were on it previously had zero vibration when going at speed.

My question is if the vibrations are a result of the mismatched / old tires or if the wheels are out of round or bent. Is there a way to tell? There is some curb rash on the front wheels, and small dents on the very outside of the lip.

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Justin
1986 Z31 NA 2+0

Hey Justin, met you the other night at RDZ

Could most likely be a combination of both the wheels anf the tires. Mismatched (and worn) tires will cause all sorts of bizarre issues from vibrations at varying speeds to tramlining to odd high frequency noises. These wheels you bought: were they stored on their side or on a car for a period of time? Could be flatspotted. Curbrash can also indicate a bent wheel.

At the very least get them balanced and it should have a tangible improvement on the issue, although it most likely won't cure it altogether. Time to spring for some new shoes

I agree with the idea of aligning and balancing them immediately and hoping it fixes the vibration.

What scares me about buying used wheels with mounted tires is I never know for sure which direction they originally rotated when they were mounted on the seller's car.

Radial tires used to be one-directional, and had arrows on them showing the intended direction of rotation the factory designed them to roll in. If you mounted them backwards and then swapped them around to roll in the reverse direction (swapping sides on the car), the belt separated from the tire very quickly. That's happened to me twice in the '70s and '80s. Both times I got a hi-speed vibration a few days before the belt let go and the tread pulled almost off the tire. Ironically it happened on the same R/F wheel, about 11 years apart.

Many tires today don't have that requirement and can be mounted to roll in either direction BUT ~ if they've rolled any significant distance (500 miles or more?) in one direction, *some* of them still cannot be swapped over to roll in the 'wrong' direction without separating. The old bias-ply tires (which came on the early cars) didn't have this problem ~ only steel belted radials are effected.

Of course, if the seller marked the tires as he removed them from his car, there's no danger of mounting them "wrong" on your car.

Another thought is if you changed wheel/tire sizes with this swap, or if you staggered the front/rear sizes. In other words, if you changed the drive angle of your Z when you mounted the new shoes. It doesn't take much to cause the driveline to spin at a different angle, which can cause a vibration in the universal joints at certain speeds.

And of course different threads on the same car can cause all sorts of weird sensations, especially if you have different threads on the same axle.

You can cut your investigative search in half by determining where the vibration is coming from. If it's all in the steering wheel, it's in the front. Try taking both hands off the wheel (when it's safe) and zee if it shakes back & forth. Also check your rearview mirror to zee if it vibrates at speed, which indicates a rear wheel or driveline problem. Work from there ~ at least you know which direction to turn when you get out of the car.

Finally, a single rubbing brake caliper will cause you all sorts of grief at certain speeds. When you feel the shake, touch the brake pedal with your foot still on the gas, and zee if it changes or goes away. Then gradually spike the handbrake to check the rear.

(That'll be 5 cents, please! )

Frank

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1970 240Z

Thanks for the helpful info as always!

As far as I know, the wheels were stored off the car from the previous owner (but no idea about the original owner or anybody in between). There were no markings on how the tires were mounted previously. Scary stuff about the belts!

The threads are mismatched on each corner so I am thinking (hoping) that is the main issue at hand.

I think that rubbing should not be an issue from what I can tell (and based on others fitting the same set of wheels on their z31s), but I will be sure to check it just to be safe.

Time to shop around for a set of tires! I am thinking of the Hankook R-S3's... unless I find a high performance tire for significantly cheaper.

Frank, I think I owe you more than 5 cents at this point... add it to the tab!
Also, I didn't realize that it was you that I met yesterday Greg!

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Justin
1986 Z31 NA 2+0

Justin,

Before you commit to new tires you may want to do two things.

One: Put the original tires and wheels back on and retest the car.

Two: Negotiate in advance of your new tire purchase, an agreement with the tire shop to check the "new" wheels for straightness.

Any tire shop equipped with a spin balancer can perform these checks.

Have them agree to mount the wheels (less tires) on their balancer. Ask to be present when they slowly spin the wheel(s) while observing both the front and back bead area for warpage, bends or dents.

A badly bent wheel will be visibly evident without the need to check it with a dial indicator etc.

Experience has shown that the Conquest/Starion wheels are prone to being bent easily as they are fairly light in weight and "positive" in their total offset from center.

Often times these wheels will check out "OK" on the outside bead, yet wind up badly "egg" shaped on the inside bead area.

Even the best new tire and perfect balancing cannot cure the harmonic vibration which occurs when the inside of a wheel is bent or out of round.

A wheel doesn't have to be perfectly true. Total wheel eccentricity of less than .060 inch will usually not show up as a vibration problem.

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Kim Blough

Thanks for the idea Kim.

It would indeed be bad to buy a whole set of tires only to find the wheels I want to mount them on is bad.

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Justin
1986 Z31 NA 2+0